What about human nature?

Humans behave differently depending upon the conditions that they live in. Even very short term changes in those conditions can change the way people behave. Most of what people refer to as "human nature" is actually human behaviour: reactions to the world around them.
Human behaviour reflects society. In a society such as capitalism, people's needs are not met and reasonable people feel insecure. People tend to acquire and hoard goods because possession provides some security. People have a tendency to distrust others because the world is organized in such a dog-eat-dog manner.
Under capitalism, and the previous systems, people have good reason to worry about tomorrow—they can lose their jobs, or be injured, or grow old, and need a cushion of wealth to fall back on. In a socialist society, everyone is entitled to have their needs met. They won't be kicked out onto the street, or forced to give up the pleasures of life. There will be no poverty. The "cushion" will be cooperatively provided by all.